Clear dummy and emptyfrozenset, so that we don't have
dangling references in case of a Py_Initialize/Py_Finalize
cycle.
diff --git a/Objects/setobject.c b/Objects/setobject.c
index e7f6e09..ff2e17f 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -978,8 +978,8 @@
 		so = free_sets[num_free_sets];
 		PyObject_GC_Del(so);
 	}
-	Py_XDECREF(dummy);
-	Py_XDECREF(emptyfrozenset);
+	Py_CLEAR(dummy);
+	Py_CLEAR(emptyfrozenset);
 }
 
 static PyObject *